U.S. application Ser. No. 121,744, filed concurrently herewith by the present applicant under the same title, discusses the hazards of lead and cadmium release from glazes and enamels containing those ingredients and describes frit compositions which are essentially free from those elements. Those frits consist essentially, in weight percent on the oxide basis, of
______________________________________ SiO.sub.2 35-47 B.sub.2 O.sub.3 5.5-9 BaO 24-42 TiO.sub.2 1.5-4 ZrO.sub.2 6-10 Li.sub.2 O 1-5 MgO 0-5 SrO 0-8 CaO 0-4 ZnO 0-10 Bi.sub.2 O.sub.3 0-8 SrO + MgO + CaO + ZnO + Bi.sub.2 O.sub.3 0-10 ______________________________________
Those frits demonstrate a coefficient of thermal expansion (20.degree.-300.degree. C.) between about 65-75.times.10.sup.-7 /.degree.C., a viscosity suitable for firing at about 700.degree.-950.degree. C., and excellent resistance to attack by acids and bases.
U.S. Ser. No. 121,744 also enumerates four vital criteria that must be exhibited by glazes and enamels and cited an optional characteristic which is required when a transparent, highly glossy decorative coating is desired:
First, the firing or maturing temperature of the frit, i.e., the temperature at which the frit will flow sufficiently to produce a smooth homogeneous coating, must be low enough to preclude thermal deformation of the substrate being coated;
second, the coefficient of thermal expansion of the frit must be compatible with that of the ware being coated to inhibit crazing and/or spalling with the most advantageous circumstance contemplating a coefficient of thermal expansion somewhat less than that of the substrate, whereby the fired coating will be placed in a state of compression when the ware is cooled to room temperature;
third, the frit must display excellent resistance to attack by acids and bases since corrosion of the coating can lead to loss of gloss, the development of haze and/or iridescence, the formation of porosity, or other effects detrimental to the appearance or physical character of the coating;
fourth, the frit must manifest good glass stability, i.e., the frit must resist devitrification during maturing; and
fifth, if a glossy decorative glaze or enamel is desired, the refractive index of the frit must be sufficiently high to produce a clear high gloss in the coating.
With regard to chemical durability, where the glaze or enamel is to be utilized in food service applications, viz, as culinary ware and tableware, it must resist attack by acids present in foods and by detergents such as are employed with commercial dishwashers.